Low energy fuse having improved properties in both axial and radial directions

ABSTRACT

A low energy fuse, comprising a plastic tube with a channel, the channel containing a reactive material able upon ignition to sustain a shock wave within the channel, the tube comprising at least two layers of plastic materials, a first plastic layer closer to the channel and a second plastic layer outside the first layer, at least the second layer containing a major amount of draw orientable polymer resin. The polymer in the second layer is axially oriented to an orientation degree (as defined) of more than 25% and less than 90% and that the polymer of the first layer has an axial orientation degree not exceeding 10% orientation degree units more than that of the second layer. The tube can be manufactured by a) forming, by extrusion of the first layer plastic, the first layer in the form of a tube, b) introducing the reactive material in the tube channel, c) limiting stretching of the first layer to give a low degree of orientation, not exceeding 10% (as defined), d) forming, by extrusion of the second layer plastic, the second layer around the first tube, while the first layer have said low degree of orientation, and e) cold-stretching the first and second layers together.

TECHNICAL FIELD

The present invention relates to a low energy fuse, comprising a plastictube with a channel, the channel containing a reactive material ableupon ignition to sustain a shock wave within the channel, the tubecomprising at least two layers of plastic materials, a first plasticlayer closer to the channel and a second plastic layer outside the firstlayer, at least the second layer containing a major amount of draworientable polymer resin. The invention also relates to a manufacturingmethod for such a fuse and the fuse manufactured by the method.

BACKGROUND

A low energy fuse of the type referred to was first described in U.S.Pat. No. 3,540,729 and numerous subsequent patents have been published.In broad terms the fuse consists of a narrow plasic tube with apyrotechnic or self-explosive reactive matter disposed within the tubechannel. The amount of reactive material is sufficient to give a highspeed shock wave in the channel, able to ignite secondary or functionalpyrotechnical devices such as detonators or transmission caps forblasting networks. Yet the amount of reactive material is sufficientlysmall to confine the reaction within the tube without destroying,disrupting or even deforming it and to make the overall device safe,harmless and noiseless in use.

Although the device is simple in principle, the physical demands placedon it are not. A substantial radial strength is needed to resist theforces produced by the shock. Signal speed is lost, or the wave halted,if the tube is substantially defomed or ruptured. Radial strength isalso needed to avoid compression and external damages and to allow crimpattachment of functional devices to the tube. Substantial axial strengthwith maintained elasticity is needed to take up forces involved inhandling, network connection and charging operations. Overall toughnessis needed to sustain the harsh field conditions before and duringblasting. Further desired properties are suitable friction propertiesand impermeability to moisture and oil.

The reactive material is typically a powder introduced in the channel.For that reason a unique constraint on the fuse tube is that theinterior surface must have suitable powder adhesion properties. A tooweak attraction may mobilize the powder, giving signal interuptions dueto material rarefactions or clogs. Too stong bonds counteracts rapidreaction and dust explosion.

The fuse tube is produced in considerable lengths and the tube materialsmust be inexpensive and the manufacturing methods cost effective.

The demands are partly contradictory and single-layered tubes tend torequire a compromise between desired properties. It has been suggestedin U.S. Pat. No. 4,328,753 to make a two-layer tube and select inner andouter materials of different properties but the materials are notoptimally used solely thereby.

It has been suggested, in for example Canadian patent 1 200 718 and U.S.Pat. No. 4,817,673 to increase axial strength by incorporatinglongitudinal reinforcing filaments in the tube material. The resultinginelastic tube is unable to absorb elongation under field conditions andtend to break or disengage from its detonator when subjected to strain.The tube material is not efficiently utilized in spite of the increasedcosts for manufacture and reinforcements.

The U.S. Pat. No. 4,607,572 describes a manufacturing method in which aninner tube with suitable adhesive properties is first manufactuted andthen elongated under overextrusion to increase manufacturing speed,minimize the amount of adhesive inner material and give an orientationto the elongated tube material. The respective materials are notefficiently used as orientation is concentrated to the inner layer,resulting in radial brittleness, whereas the outer layer contributeslittle to axial strength. A product inclined to fracture will resultunless stretching is limited.

The European patent specification 327 219 describes a single-layer tubeextruded from a mixture of a draw orientable polymer and a minor amountof a polymer of adhesive quality. In manufacture the adhesive polymer issaid to concentrate at the inner surface of the tube and substantialorientation of the polymers is imposed in a cold stretching stepfollowing extrusion. The orientation adds substantial axial strength tothe tube but the radial strength is lost proportionally, again resultingin a poor resistance to the shock and a poor utilization of the inherentstrength capacities of the polymers is used.

These more advanced tube designs add to production costs and problems.Simple coextrusion or overextrusion can be done fairly easily but do notutilize the full strength of the materials. Orientation by substantialstretching can he run efficiently but tend to give unacceptable radialproperties. Limited stretching may be preferred but tends to giveunstable process conditions and final tube properties unless the draworientable polymer is supported by other layers or conditions.

The Invention Generally

A main object of the present invention is to avoid the problems ofhitherto used fuse tubes. A more specific object is to provide a fusetube of two or more layers with optimized utilization of materialstrength properies. Another object is to provide a fuse tube havingsuitable strenght properties both axially and radially. Yet anotherobject is to provide a fuse tube having an inner layer giving a majorcontribution to radial strength and an outer layer giving a majorcontribution to axial strength. Still another object is to provide amultiple layer tube further improved properties in the shove respects. Afurther object of the invention is to provide a suitable manufacturingmethod for the fuse tube, giving the desired properties. A furtherobject is to provide a tube manufacturing method avoiding problems withdesired process parameters.

These objects are reached with the characteristics set forth in theappended claims.

According to the invention a low energy fuse tube, of the type firstsaid herein, is provided in which the polymer in the second layer isaxially oriented to between 20 and 90% of its available orientation (asdefined) and that the polymer of the first layer has an axialorientation not more than 10% more than that of the second layer.

The axial orientation of the second layer polymer gives improved axialstrength to the tube. The orientation is limited in order to maintain anaxial elastic and plastic elongation capability of the tube to meet theabovesaid fabrication and field condition requirements. The orientationis also limited in order to retain a significant radial strengthcontribution to the tube and to avoid brittleness. The orientation ofthe first layer polymer is at most substantially the same as in thesecond layer. To the extent it is in any way higher it is only due tounavoidable elongation in certain manufacturing alternatives. Preferablythe first layer orientation is lower than that of the second layer,especially at higher orientation degrees in the second layer. The loworientation degree of the first layer contributes to radial strength ofthe tube. Above all the layer hereby maintains non-brittle properties inthe first layer itself both radially and axially, which has been foundoptimal for tube resistance to the shock wave forces. Without beingbound by theory it is believed that observed lack of tube resistance toshock wave is due to notch weaknesses, caused by the sudden shock waveon a polymer layer brittled in any direction by stretch orientation, andbeing especially fatal when crack formations are easily initiated in theinner layer. The present suggestion maintains first layer toughness inboth radial and axial directions. Radial properties are directlysignificant for shock expansion forces, but also axial crack tendenciesare important among others since tube failures are mainly found at kinksand folds on the tube. The low inner layer orientation is complementedby the outer layer being chiefly responsible for tube overall axialstrength. The low inner layer orientation is also consistent with therequirement for good adhesion properties at the tube channel. Accordingto preferred embodiments of the invention the tube may comprise furtherlayers, amplifying the abovesaid basic properties or adding beneficialsecondary properties to the tube.

The invention also relates to a method for manufacture of the abovesaidtube, comprising the steps of a) forming, by extrusion of the firstlayer plastic, the first layer in the form of a tube, b) introducing thereactive material in the tube channel, c) limiting stretching of thefirst layer to give a low degree of orientation, not exceeding 10% (asdefined), d) forming, by extrusion of the second layer plastic, thesecond layer around the first tube, while the first layer have said lowdegree of orientation, and e) cold-stretching the first and secondlayers together, preferably to a limited degree of oientation in thelayers.

The final cold stretching step provides the desired orientation of atleast the second layer for improved axial tube strength. The limitedstretching degree gives maintained elongation properties to the tube aswell as a radial strength contribution from the first layer. By limitingstretching of the first layer before and during application of thesecond layer the first layer will not be overly oriented in the laststretching step but will have retained non-brittle properties bothradially and axially. Stretching the layers together assures that thefirst layer orientation will not exceed that of the second layer andfurther serves to facilitate the stretching operation itself asdifferent material properties of the layers tend to smooth outirregularities and instabilities. Additional steps in the method may beused to further limit final orientation in the first layer andconcentrate orientation to the second layer. The method may include afurther step wherein one or more additional layers are provided, beforeor after the stretching operation, in order to achive the additionaladvantages outlined. The method can generally be implemented inco-extrusion, over-extrusion and tandem extrusion process layouts.

Further objects and advantages with the invention will be evident fromthe detailed description hereinbelow.

Definitions

By "stretch ratio" shall be understood the weight to weight ratio ofequal lengths of tube before and after stretching respectively. Themeasure is substantially similar to the length ratio of the same part oftube after and before stretching, but also includes the density change.

"Cold-stretching" refers to stretching under conditions resulting insubstantial molecular orientation in draw orientable polymers. Theconditions may require a temperature below the solidificationtemperature for the polymer, as opposed to warm-stretching that allowsfor substantial molecular relaxation simultaneous with the stretching.Unless otherwise indicated, second layer conditions are referred to asthe invention aims at a concentration of orientation to this layer.

"Plastic" refers to the total material composition used for a layer. Itincludes a main part of one or more "polymers", providing layer strengthand generally able to accept orientation, as well as any additive otherthan the polymers.

"Co-extrusion" refers to the process of substantially simultaneousformation of at least two layers, normally by extrusion of the meltsfrom different orifices on the same die head. "Over-extrusion" refers tothe process of first forming a layer in sufficient consolidated form forallowing the extrudate to be fed through a second extruder in which asecond layer is applied. "Tandem extrusion" refers to an over-extrusionprocess in-line with the first extrusion, without intermediate storageof the first formed extrudate.

"Fold test" refers to a method for determination of the the fuse tubetendency to rupture under the influence of the shock at folds providedon the tube. On a continuous length of the tube to be tested a number offolds are provided with a minimum length of 50 cm tube between thefolds. The folds are fixed in place by insertion into circular holesdrilled in a plate to a depth of about 3 cm and with a diameter of about65% larger than the double diameter of the tube (or about 8 mm for 3 mmtubes). After tube initiation the folds are inspected for any kind ofwall rupture and the result is given as the quotient between the numberof folds with rupture to the total number of folds.

"Orientation degree" refers to a value on the orientation in the axialdirection of the fuse tube and is expressed in percent, zero percentrepresenting no orientation or random orientation and one hundredpercent representing maximum possible orientation for the polymer underconsideration. To establish an actual sample value between theseextremes different methods may be employed. The methods suggested hereinare either based on "yield strength" or "IR-spectrometry".

The "yield strength" method for orientation degree measurementsestablishes an elastic tensile strength value for the sample, expressedin force per sample cross-section area (E, MPa), by stretching thesample under recording of elongation (L, m), force and cross-section. Atypical specimen will first elongate elastically under rapid increase ofapplied force, then elongate with plastic deformation under slowerincrease of applied force. The force at the "knee" formed between thesephases is taken as the maximum elastic strength of the sample whenexpressed in relation to its cross-section at that point. The value isdetermined for the oriented sample under study (Ex) as well as for thesame material unoriented (Emin) and fully oriented (Emax) and theorientation degree is given by (Ex-Emin)/(Emax-Emin)*100. FIG. 4illustrates typical graphs of L versus E and extraction of abovesaidvalues. This method is simple but requires access to an isolated sampleof the tested material and of its unoriented and fully orientedcounterparts.

The "IR-spectrometry" method for orientation degree termination measuresabsorption of polarized IR-radiation on one or several sample molecularvibration frequencies affected by orientation and gives strongabsorption with the polarizing plane parallel to the vibration and weakabsorption in the perpendicular plane. Absorbance (A, dimensionless) isnormally determined with the polarization direction of the radiationparallel and orthogonal to the stretch axis for the sample and adichroism ratio (D, dimensionless) is determined as the ratio betweenthese absorbancies. An orientation factor (f, dimensionless) can becalculated, varying between 0 and 1 for none to full orientation alongthe selected axis. This value is used herein in percentage form fororientation degrees along the stretch axis. The method may give absoluteorientation degree measurements, normally used in conjunction with FTIR(Fourier Transform Infrared Spectroscopy), and is described e. g. InEncyclopedia of Polymer Science and Engineering, edition 1988, volume14, pages 542(564)-576, by H F Mark et al.

The Fuse Tube Generally

Although the tube of the present invention may have utility for otherpurposes than described, it is preferred to use it in connection withthe low energy fuse type referred to in the introduction and in thepatents cited. It is a characteristic of such a fuse that the tube is anarrow plastic tube and still able to confine the shock in its interiorwith maintained structural integrity.

The outer diameter of the tube may be between 1 and 10 mm and typicallybetween 2 and 5 mm. The inner diameter may be between 0.2 and 4 mm andespecially between 0.5 and 3 mm. Commercial products tend to have anouter diameter around 3 mm and an inner diameter around 1 mm. The tubemay have any cross-sectional shape but is preferably circular.

The reactive material can be self-explosive compounds, such as PETN,RDX, HMX etc., optionally with some additive for improved initiabilitysuch s aluminum. Signal speed with this kind of materials generally liesbetween 1000 and 3000 m/sec. The reactive material can also be apyrotechnical mixture of fuel and oxidizer components, normallygenerating little gas during reaction therefrom. Mixtures of this kind,mainly intended to retard signal speed for delay purposes, are describedfor example in U.S. Pat. Nos. 4,660,474, 4,756,250 and 4,838,165 and inEuropean patent specification 384 630 and PCT specification 81/03954.Signal speed may be between about 500 an 1500 m/sec. The reactivematerials are generally pulverulent with a particle size between about 1and 100 micrometers, in particular between 5 and 50 micrometers. Thematerial is preferably adhered to the channel wall as described but mayalso be attached to a bearer in the channel as in the cited U.S. Pat.No. 3,590,739 or introduced in fiber form as in U.S. Pat. No. 4,290,366.

The necessary amount of reactive material in the channel is kept as lowas possible for stable reaction at the desired speed without disruptingthe tube. The absolute amount depends on the nature of the reactivematerial as well as the size of the tube. As a non-limiting example, forthe commercial type product with a self-explosive material, the amountmay be be between 1 and 100 mg/m or preferably between 5 and 50 mg/m.

The tube of the invention can have these or similar characteristics. Itshall comprise at least two layers, a first layer closer to the channeland a second layer outside the first layer. It is preferred that the twolayers consist of different materials as will be explained. Each of thedescribed layers of the tube can internally be divided in severaldifferent layers, with discrete or continously varying properties. Thetube may include reinforcing fibres but prefereably such fibres areomitted as superfluous or even detrimental to desired axial resilience.

The First Layer

The main polymer of the plastic in the first layer shall have a limitedorientation degree to meet the abovesaid objectives. In some instancesthe orientation degree may he higher than that in the second layer, forexample if a certain stretching of the first layer has to he done forprocess reasons in an overextrusion process before finalcold-stretching. The orientation degree should then be kept less than10% more than that of the second layer. Otherwise the upper limitationon the orientation degree for the first layer is that it shall not behigher than the orientation degree of the second layer (to be specifiedhereinafter). Preferably the orientation degree is lower and mostpreferably substantially lower, especially at higher orientation degreesin the second layer. In absolute terms the orientation degree can bebelow 35%, preferably below 25% and more preferably below 15%. it ispreferred that the inner layer has a minimum amount of axialorientation, e. g. an orientation degree exceeding 5% and also exceeding10%.

General methods for securing a low orientation in the first layer may heto have a higher absolute temperature in the first layer than in thesecond layer during cold-stretching, to have a higher relativetemperature by using a polymer having lower softening or melttemperature than the polymer of the second layer, by using a lessorientable polymer, e.g. more branched or of less density, than thepolymer of the second layer.

The first layer is preferably the innermost layer and should preferablyhave suitable powder adhesion properties as described. The adhesionmechanism can be of different nature, such as pure tack or electrostaticattraction. A prefered way is to use a polymer containing polarfunctional groups giving dipolar attraction with maintained goodstrength properties of the polymer. A preferred polymer type is ionomerssuch as Surlyn and Primacore (registered trade marks). Furthersuggestions for polar type polymers are given in the abovesaid Europeanspecification 327 219.

In case the limited orientation degree of the first layer is due to alower orientability of that polymer, polymers can be selected havingbranched structure and a lower density, such as between 850 and 950 orbetween 880 and 925 kg/cu.m. for polyethylenes and correspondingdensities for other polymers.

The Second Layer

The second layer adds to tube axial strength and should have a notableorientation degree, such as above 20%, preferably above 30% and morepreferably above 40%. The maximum orientation degree can be up to 90% incase the second layer is not solely reponsible for tube externalbrittleness, for example if a further layer is arranged on the secondlayer. Otherwise, and for best overall properties, the orientationdegree should be limited to less than 80% or even 70%. This means thatthe second layer will have an intermediate orientation degree whencompared to products drawn to maximum tensile strength.

The material for second layer shall be selected from draw orientablepolymers with significant durability and strength. Linear polymers areto be preferred, such as fibre forming polymers. Any density type can beused although it is preferred to select polymers corresponding topolyethylenes in between of LDPE and HDPE, such as LLDPE, LMDPE etc.Metric densities can be between 900 and 1000 and especially between 925and 975 kg/cu.m. Corresponding polymers of other monomers than ethylenecan be used, such as propylene or copolymers therebetween. Non-olefinicpolymers are also usable such as polyamides or polyesters. Furthersuggestions are given in the abovesaid European specification 327 219.

Concentration of orientation to the second layer can be facilitated byselecting the more easily draw orientable polymers. Another approach isto select a polymer with high softening temperature. The temperatureshould then be higher than the softening temperature for the polymers ofthe first layer. A suitable softening temperature could be above 100° C.and preferably above 120° C.

The second layer can be the outermost part of the tube but it is alsopossible and sometimes preferable that the tube comprises furtherlayers.

Further Layers

The tube may have further layers than the first and second layers.Additional layers can be used predominantly to add secondary propertiesto the tube or can be part of the structural strength properties of thetube.

An additional innermost layer can be used to give the channel surfacepowder adhesive properties, although it is preferred that the firstlayer fullfils this function. Such an additional layer should be keptthin, e.g. below 0.4 mm and preferably below 0.2 mm and most preferablybelow 0.1 mm in thickness.

An additional outermost layer can be used for example as a barrier formoisture or oils or to make the tube surface smooth, soft or colored.

A preferred additional layer for structural strength purposes is toprovide a plastic third layer outside the second layer. Similar to thefirst layer, the third layer preferably has a limited orientationdegree, not exceeding 10% more then the second layer, preferably havinga less orientation degree than the second layer and more preferably loworientation degree, not exceeding 35%, and preferably not exceeding 25%.Some orientation is desirable in the third layer, e.g. above 5% andpreferably above 10%.

The material may be a draw orientable polymer of the second layer type.Other alternatives are EVA, EAA polyamides etc. To avoid too highorientation degrees in the third layer, less orientable plastics couldBe selected. Alternatively, or in addition, the polymer selected couldhave a lower softenining temperature than the polymer of the secondlayer.

The Final Tube

The size relationship between the layers can vary depending on thenumber of layers involved and the relative strength of the materailsinvolved and their given orientation degrees within the general limitsof the invention. In general terms the second layer provides axialstrength, due to the stretch orientation, and also provides asignificant contribution to radial strength, due to the appliedlimitations on said orientation. The first layer provides radialstrength but above all prevents initiation of cracks or notch weaknessfailures in the critical inner parts of the tube.

Based on this division of contributions, the first layer size can bekept small, e.g. less than 50% and preferably less than 35% of tube wallcross-section area but exceeds 10% and preferably 15% of said area. Inabsolute terms the first layer wall thickness can be below 0.4 mm andpreferably below 0.3 mm but exceeds 0.1 mm and preferably exceeds 0.2mm.

The remaining part of the wall size should be made up of the secondlayer in case of two-layer tubes, disregarding here any thin additionallayer for secondary purposes. In case an outermost third layer isprovided for structural strength purposes, such a layer preferablycontributes considerably to radial strength. The second layer could thenbe reduced in size, e.g. to between 20% and 60% and preferably between30% and 50% of tube wall area and the third layer could also be given asize within these limits. The orientation degree in the second layercould then also be increased, as specified, to make a greatercontribution to axial strength and less to radial.

Overall strength of the tube should exceed 25 MPa, preferably exceeds 40MPa and most preferably exceeds 50 MPa.

Due to stretching and orientation the final tube is inclined to shrinkunder relaxation. Under ambient and operational temperatures theshrinking is limited due to the equalizing influence of the interferinglayers and effected stress relaxation, typically below 5% an preferablybelow 3%. Heat shrinking, though, may exceed 3% and also exceed 5% inlength.

The Extrusion Operations

The manufacturing method should avoid a high degree of orientation inthe first layer and concentrate orientation to the second layer. Sincethe suggested way of orienting the second layer polymer is tocold-stretch the first and second layers together, a first requirementon the manufacturing method is to secure a low degree of orientation inthe first layer of the combined first and second layer tube, before thecold-stretching step. Generally this means that any kind ofcold-stretching of the first layer should be avoided before the secondlayer is applied. In some manufacturing methods, like in overextrusion,some elongation of the first layer is unavoidable, so the method shouldallow for a limited degree of orientation in the first layer, beforecold-stretching, say below 20% and preferably below 10%. In othermethods, like coextrusion, virtually no orientation needs to beintroduced in the layer forming steps.

Although the reactive material can be introduced in a ready tubechannel, at any point between formation of the consolidated inner tubeand the final tube, by for example blowing or sucking a pulverulentmaterial or feeding a liquid with the reactive material through discretelength of tube, it is generally preferred to introduce the reactivematerial continuously during formation of the inner layer or layers ofthe tube. This can be done by feeding and dispensing the reactivematerial through a channel or nozzle in the extrusion head for the innerlayer, arranged centrally in relation to the annular die opening for theextrudate. Normally this means that the material is introducedessentially simultaneously with the formation of the inner layer.

Over-extrusion or tandem extrusion starts with the extrusion of thefirst layer in tube form followed by at least some cooling tosolidification before the second layer is applied in a second extrusionstep. As said, some elongation may inevitably be required during feedingof the first layer tube through the overextruder head but otherwise noorienting by stretching should take place, neither before nor duringoverextrusion. This does not exclude warm or melt stretching of theinner layer but it is preferred to extrude the melt in a larger than thedesired cross-section and draw the melt down before solidification. Apreferred draw-down degree, expressed as diameter ratio, may be between2 and 10 times and preferably beween 3 and 5 times. Consolidation mayrequire cooling below the solidification temperature of the plasticmaterial. In order to limit orientaiton of the layer, in this and thefollowing steps, it is desirable to maintain a relatively hightemperature of the layer. Preferably the first layer-tube is not cooledto less than 25° C., and preferably not less than 15° C., below itssolidification temperture before overextrusion. Temperature adoption mayrequire a cooling step, e.g. in a tandem process, or a heating step,e.g. in an overextrusion process.

The overextrusion step in the second extruder head is not highlycritical. Draw down of the melt may take place as in the first step. Ifthe tube shall comprise a third layer as described, it is preferred toco-extrude it simultaneous with the second layer although it iscertainly possible to apply the third layer in a separate overextrusionstep following the second layer extrusion, with optional cooling orheating steps in between. The third layer will be stretched togetherwith the other layers and a desired limitation of orientation degree inthis layer may be accomplished by any of the general methods described.Generally the two step extrusion processes give good temperature controlover the layers in the production process.

Co-extrusion of first and second layers substantially simultaneously,for example from different nozzles in the same die head, is a simplemethod also resulting in a low initial orientation degree in the firstlayer when compared to the second layer and any orientation introducedduring or after this step will affect both layers and not only the firstlayer. A melt draw down ratio may preferably be used as describedbefore. If a third layer is to be applied, it can preferably be doneessentially simultaneously, e.g in the same extrusion head to give atriple-extrusion step, although it is also possible to arrange aseparate extrusion step after the coextrusion step, with optionalcooling or heating steps in between. Here again the third layer will bestretched together with the first and second layers and orientation canbe limited with the same general actions.

Further layers may also be applied after the cold-stretching step. Thisin particular for layers intended for secondary properties but alsolayers for structural purposes. A third layer provided in this way mayfor example be given a very low degree of orientation.

The Stretch Operation

As indicated some degree of orientation may result from stretchingduring the extrusion operations and thereafter. It is preferred,however, that most of the cold-stretching is made under controlledconditions in a separate stretching zone. Such a zone may include atleast two gripping means for the tube, e.g. opposed endless belts orcapstan wheels, the second gripping means being driven with higher speedthan the first, thereby elongating the tube in a controlled manner.

The tube must have a certain rigidity to sustain forces from thegripping means without deformation and accordingly should have atemperature well below its softening temperature when passing thegripping means, such as below 50° C. or even below 40° C., subject tothe nature of the plastics employed. A cooling step may be requiredbefore the first gripping means and also before the second grippingmeans, especially if the stretch zone in a preferred manner includes aheating zone.

In general terms a draw orientable polymer under elongation tend toroughly maintain its axial tensile strength in spite of the decreasingcross-section. When reaching a maximum degree of orientation furtherelongation tends to break the material. According to the invention thesecond layer shall be given an intermediate degree of orientation, i.e.a significant orientation although well below the maximum possible. Thestretch ratio for this purpose depends on the polymer used and thestretch conditions employed. Roughly the stretch ratio should exceed 1.5and preferably exceeds 2 but could be kept under 5 and preferably alsounder 4.

Draw orientable polymers also tend to elongate at a well defined andlocalized "neck-in" point on the drawn material, which usually causes noprobelms, especially not at high elongation ratios. At intermediatestretch ratios, however, the neck-in point may fluctuate both inposition and shape and if so it is preferred to stabilize the process bysmoothing out or extending the neck-in point, e.g. to more than 10 cmand preferably to more than 25 cm of the steep part of the neck.

It is preferred to include a heating step in the stretch zone for theabovesaid purpose and for obtaining a uniform orientation structure.Good results have been obtained by raising tube temperature to betweenthe amorphous and crystalline melting points for the second layerpolymer or generally to a temperature between 5° and 25° C. below thesecond layer plastic softening point.

It is further preferred to use an axially extended heating zone and touse surface heating, for example in an oven or heating bath.

A single step stretching operation is suitable and most convenientalthough it is possible that several streching steps as described areused.

Abovesaid conditions are selected to give orientation primarily in thesecond layer. Less orientation in the first layer may be accmplished byusing a first layer polymer having a lower melt temperature than that ofthe polymer in the second layer. For lowest orientation of the firstlayer stretching should be conducted above the softening temperature forthe first layer but below the softening temperature for the secondlayer, although improvements have been experienced also at elevatedtempertures slightly below the softening point for the first layer.Another approach, useful also at small differences in said softeningtemperatures, is to maintain a higher absolute temperatue in the firstlayer and a lower absolute temperature in the second layer, for exampleby cooling the tube from the exterior side immediately beforestretching. A less draw orientable polymer in the first layer than inthe second layer also assists in reducing orientation in the firstlayer.

When the tube comprises a third layer it is preferred to have a lowerorientation degree in that layer than in the second layer. The sameprincipal methods as for the first layer can be used to reduceorientation degree in this layer. A higher absolute temperature in thethird layer than in the second layer can be obtained for example byheating the tube from the exterior side immediately before stretching.

The stretch operation builds stresses into the tube, making the tube aptto relax. Molecular orientation is intentionally introduced and shallnot be relaxed, although it may be unmasked as shrinking at asubstantial temperature raise. To avoid shrinking at ambient or nearambient temperature, stress relaxation can be done with advantage beforeuse of the tube, preferably at a slight temperature increase under lowtension, which can be done in an idle loop in-line.

SUMMARY OF DRAWINGS

FIGS. 1A and 1B schematically show layer structure and orientationpattern of prior art fuse tubes with two and three layers respectively.

FIGS. 2A and 2B schematically show layer structure and orientationpattern of preferred fuse tubes with two and three layers respectivelyaccording to the invention.

FIG. 3 shows in schematic form a preferred general process layoutaccording to the invention for manufacture of two and three layer tubes.FIG. 3A relates to the extrusion operations and FIG. 3B relates to thestretching operation.

FIG. 4 illustrates an elongation versus elastic tensile strength graphand values needed for orientation degree calculations.

DESCRIPTION OF DRAWINGS

FIG. 1A illustrates a stretched two-layer tube of the prior art kind, inwhich an inner layer is first produced and overextruded under stretchingin a second step. The arrows indicate the resulting general orientationpattern, showing significant orientation of the inner layer andsubstantially no orientation in the outer layer. FIG. 1B illustrates aprior art tube in which a two-layer tube of the kind shown in FIG. 1A issubmitted to a further overextrusion step under elongation. Theinnermost layer has still a more pronounced orientation, theintermediate layer clearly less and the outermost layer substantiallynil.

FIG. 2A illustrates the two-layer tube of the invention as described.Orientation is concentrated in the second, outer, layer while the first,inner, layer has substantially less orientation. FIG. 2B illustrates athree-layer tube of the invention, having inner layers corresponding tothose shown in FIG. 2A and a third, outermost layer with a low degree oforientation. It should be noted that the third layer can be providedwithout further increasing or affecting the orientation properties ofthe two inner layers, contrary to the prior art. In both embodiments ofthe invention the second layer provides axial strength, and non-brittleand shock-resistant properties are maintained in the first layer. Herebythe layers contribute optimally to desired fuse tube properties and theinherent strength capacities of the polymers are better utilized than inthe prior art products.

FIG. 3A illustrates schematically the extrusion process for two andthree layer tubes respectively. The first, second and third lyers areshown at positions 1, 2 and 3 respectively. In the two layer process thefirst layer 1 may be extruded from a first extruder 4 and the secondlayer 2 may be applied in a second extruder 5. Extruder 4 is optionalinsofar as extruder 5 co-extrudes both the first layer 1 and the secondlayer 2 simultaneously. In the three layer process the layers 1, 2 and 3may be extruded from three different extruders 6, 7, and 8. Both theextruders 6 and 7 are optional insofar as extruder 8 is atriple-extruder and one of them may be omitted if extruder 8 is a singleor double layer overextruder. All optional extruders represent bothin-line tandem and overextrusion processes and some form of coolingsteps are normally required between extruders. The extrusion processresults in a two or three layer tube 9 to be transferred to thesubsequent stretch operation.

FIG. 3B illustrates the preferred stretch operation in which tube 9 fromthe extrusion operation is drawn to a narrower tube 10. The still hottube from the extruders is cooled in a cooler 11 to a sufficiently lowtemperature to sustain the first gripping means of capstan type 12,representing the first end of a stretching zone ending with capstan 15,driven at higher speed than capstan 12. The tube is reheated in heater13 of, for example, oven or hot bath type. The main part of tubestretching, imposed by the speed difference between capstans 12 and 15,takes place on the softened tube in this heater. The drawn tube 10 ofreduced diameter is fed into a cooler 14 in which the temperature isreduced, at least to the degree required for passage of the secondcapstan 15. Stresses in the tube 10 may be released under viscoelasticshrinking in an optional relaxation step 16, in which the tube isallowed to loop under low tension and slightly elevated temperature.

The graph of FIG. 4 and its use for orientation degree calculations hasbeen described in connection with the definition of "orientationdegree", "yield strength" method, under the Definitions heading.

EXAMPLE 1

A reference two layer tube without cold-stretching was prepared withfirst innermost layer of Surlyn 8940 (Teg. Trade Mark of DuPont) and anoutermost second layer of linear low density polyethylene (NCPE 8706,from Neste Polyethylene AB). The two layers were coextruded at 200° C.for the first layer material and at 210° C. for the second layermaterial from a common extrusion tool, fed with separate screws in aboutequal volume rates, into a common annular slit of 13.5 mm outer diameterand 6.5 mm inner diameter. The extruded melt was drawn down in meltcondition to an outer diameter of 2.6 mm and was fed into a coolingmanifold fed with cooling water of about 25° C. in which the tube wascold to about 40° C. before collected at a spool. The tube had a roomtemperature tensile strength at break of 105N. The fold test result was382/400 (ruptures/number of folds).

EXAMPLE 2

A two layer tube yes prepared as in Example 1 and with the samematerials in the first and second layers extruded at the same volumerates from the same extrusion tool. Simultaneously reactive material ofHMX/Al in a weight ratio of 92/8 and in an amount of about 36 mg/m wasfed into the tube interior from a centrally arranged canula in theextrusion tool. The extruded tube yes drawn down to 3.6 mm outerdiameter in melted condition and yes cooled in the same manner as inExample 1 to about the same temperature. The cooled tube was fed into astretch zone between two capstans and initially heated for about 14seconds in a water bath of 74° C. and cold-stretched in a stretch ratioof 2:1. The tube is again cooled to about 50° C. before passage of thesecond capstan. The so prepared tube was collected on spool. Tube roomtemperature tensile strength at break was 200N and fold test result111/400.

The tubes of Examples 1 and 2 were intentionally given a thinner thanrequired wall thickness to show a higher than normal failure rate in thefold test, in order to amplify differences present.

EXAMPLE 3

A three layer tube was manufactured, consisting of a first layer ofSurlyn 8940, a second layer of a LMDPE (NCPE 1935, from NestePolyethylene AB) and a third layer of LLDPE (NCPE 8706), the weightrelation between the three layers being about 33/40/25 in the finaltube. The first and second layers were co-extruded at 205° C. and 220°C. respectively from the same extruder tool as described in Example 1.The melt was drawn down to an outer diameter of 3.6 mm before cooling ina water bath of 15° C., giving a temperature of 40° C. on the excitingtube. The cooled tube was dried in a vacuum drier and further dried andheated in a hot air blower step, giving an approximative temperature of45° to 50° C. The tube was fed in-line directly through an overextrusionstep, wherein the third layer plastic was applied at about 210° C. Thethree layer tube was cooled and stretched as described in the previousexamples at a temperature of 98° C. After cooling before the secondcapstan, the tube was stress relaxed for about 20 seconds in a lowtension loop accumulator at a temperture of about 100° C. provided by aninfra heater. The tube was cooled, dried and collected at about ambienttemperature. Tube tensile strength as above was 230N and the fold testresult 0/400.

We claim:
 1. A low energy fuse comprising a plastic tube with a channel,the channel containing a reactive material able upon ignition to sustaina shock wave within the channel, the tube comprising at least two layersof plastic materials, a first plastic layer closer to the channel and asecond plastic layer outside the first layer, at least the second layercontaining a major amount of draw orientable polymer resin wherein thepolymer in the second layer is axially oriented to an orientation degreeof more than 20% and less than 90% and that the polymer of the firstlayer is axially oriented to an orientation degree not exceeding 10%orientation degree units more than that of the second layer with saidorientation degrees being expressed as percentages of the maximumpossible orientation in the axial direction for each of said layers. 2.The fuse of claim 1, wherein the orientation degree of the first layeris not more than the orientation degree of the second layer.
 3. The fuseof claim 2, wherein the orientation degree of the first layer is lessthan that of the second layer.
 4. The fuse of claim 3, wherein theorientation degree of the first layer is below 35%.
 5. The fuse of claim1, wherein the orientation degree of the first layer is above 5%.
 6. Thefuse of claim 1, wherein the melt temperature of the polymer in thefirst layer is lower than the melt temperature of the polymer in thesecond layer.
 7. The fuse of claim 1, wherein the polymer of the firstlayer is less draw orientable than the polymer of the second layer. 8.The fuse of claim 1, wherein the plastic material of the first layercontains polar groups.
 9. The fuse of claim 8, wherein the polymer ofthe first layer contains an ionomer.
 10. The fuse of claim 1, whereinthe first layer comprises several individual layers.
 11. The fuse ofclaim 1, wherein the first layer is the innermost layer of the tube. 12.The fuse of claim 1, wherein the second layer has an orientation degreebetween 25 and 90%.
 13. The fuse of claim 12, wherein the second layerhas an orientation degree between 25 and 60%.
 14. The fuse of claim 13,wherein the second layer size is not less than 60% of the tube wallcross-section area.
 15. The fuse of claim 12, wherein the second layerhas an orientation degree between 50 and 90%.
 16. The fuse of claim 15,wherein the second layer size is less than 60% of the tube wallcross-section area.
 17. The fuse of claim 1, wherein the melttemperature of the second layer polymer is higher than 120° C.
 18. Thefuse of claim 1, wherein the plastic of the second layer contains amajor amount of a linear polymer.
 19. The fuse of claim 17, wherein thedensity of the polymer corresponds to a range between that of standardLDPE and HDPE.
 20. The fuse of claim 1, wherein the second layercomprises several individual layers.
 21. The fuse of claim 1, whereinthe second layer is the outermost layer of the tube.
 22. The fuse ofclaim 1, wherein the tube comprises a third plastic layer outside thefirst and second layers.
 23. The fuse of claim 22, wherein the polymerorientation degree of the third layer is the same or less than theorientation degree of the second layer.
 24. The fuse of claim 23,wherein the orientation degree in the second layer is at least 10%higher than in the third layer.
 25. The fuse of claim 22, wherein thethird layer is the outermost layer of the tube.
 26. The fuse of claim22, wherein the melt temperature of the polymer in the third layer islower than the melt temperature of the polymer in the second layer. 27.The fuse of claim 1, wherein the polymer of the third layer is less draworientable than the polymer of the second layer.
 28. The fuse of claim22, wherein the polymer of the third layer is selected from the groupconsisting of EVA, EAA and LLD.
 29. The fuse of claim 22, wherein thethird layer comprises several individual layers.
 30. The fuse of claim25, wherein the third layer orientation degree is less than 35%.
 31. Thefuse of claim 1, wherein the tube has a cold relaxable axial shrinkingof less than 3%.
 32. The fuse of claim 1, wherein the tube has a heatrelaxable axial shrinking of more than 3%.
 33. The fuse of claim 1,wherein the tensile strength of the tube is above 40 MPa.
 34. The fuseof claim 1, wherein the outer diameter of the tube is between 1 and 10mm.
 35. The fuse of claim 1, wherein the inner diameter of the tube isbetween 0.5 and 3 mm.
 36. A low energy fuse comprising a plastic tubewith a channel having a tensile strength above 40 MPa, the channelcontaining a reactive material able upon ignition to sustain a shockwave within the channel, the tube comprising at least two layers ofplastic materials, a first plastic layer closer to the channel and asecond plastic layer outside the first layer, at least the second layercontaining a major amount of draw orientable polymer resin wherein thepolymer in the second layer is axially oriented to an orientation degreeof more than 20% and less than 90% and the polymer of the first layer isaxially oriented to an orientation degree more than 5% and less than35%, with said orientation degrees being expressed as percentages of themaximum possible orientation in the axial direction for each of saidlayers and with the proviso that the orientation degree of said firstlayer is less than that of said second layer.